1. Field of the Invention
The present invention generally relates to an optical pickup, and particularly relates to an optical pickup used for detecting a focusing error by an astigmatism method.
2. Background Art
There exists an apparatus for recording information onto an optical information recording disk (hereinafter, merely referred to as an optical disk) or reading recorded information from the optical disk. In such an apparatus, an astigmatism method is known as one of the methods of detecting a focusing error of a light beam irradiated from a pickup for recording/reading information.
As the optical pickup used for detecting the focusing error by this astigmatism method, there has been conventionally proposed such an apparatus as shown in FIG. 1. In the drawing, light emitted from a light source 1 such as a laser diode, or the like, enters a half beam splitter 2 and is partially deflected by its reflection surface. Then, the thus deflected light radiates through an objective lens 3 onto a recording surface of an optical disk 4 as a focused beam. The light reflected from the optical disk 4 in response to the light irradiated onto the optical disk 4 is converged by the objective lens 3 and transmitted through the half beam splitter 2. At this time, astigmatism is hardly generated in the light transmitted through the half beam splitter 2 because the light-incident and emitting surfaces of the half beam splitter 2 are set perpendicularly to an optical axis of the reflection light.
The light transmitted through the half beam splitter 2 is made astigmatic by its passage through a cylindrical lens 5 and thereafter impinges onto an optical detector 6. Being astigmatic, the shape of the luminous flux imaged on a light-reception surface of the optical detector 6 is changed depending on the positional relationship between the recording surface of the optical disk 4 and a convergence point of the light irradiated onto the disk. In order to detect this change in shape of the luminous flux, the optical detector 6 is composed of four individual elements disposed as if the light-reception surface is divided into four parts divided by two straight lines perpendicular to each other. The optical detector 6 is arranged such that its light-reception surface is disposed at the position where the shape of the luminous flux becomes circular when it is focused (a focusing error is zero). A sum of respective output is obtained for each pair of elements disposed opposite to each other with respect to a center of the light-reception surface. A differential output between the thus obtained respective sums of the two pairs of elements is derived as a focusing error signal. If the optical pickup and the optical disk 4 are in a predetermined relationship, the luminous flux at the optical detector 6 is circular and the difference of the sums is zero. However if the optical disk 4 is displaced in either direction, the difference of the sums is non-zero with the sign of the difference indicating the direction of displacement.
In the thus arranged conventional pickup, however, there has been such a disadvantage that much time is consumed for producing the half beam splitter 2 because it is necessary to fabricate the half beam splitter 2 by sticking glass optical elements or the like to each other, resulting in an increase in cost as well as increase in weight of the apparatus.
Further known is another conventional apparatus, shown in FIG. 2, proposed to eliminate the foregoing disadvantage. The apparatus of FIG. 2 employs a parallel-plane beam splitter 7 which is provided with light-incident and emitting surfaces arranged to be inclined relative to an optical axis of reflection light.
In the thus arranged conventional apparatus, astigmatism and the like are given to light by the beam splitter 7 when the light is passed through the latter because the light-incident and emitting surfaces of the beam splitter 7 are arranged to be inclined relative to an optical axis of converged light. Therefore, it is unnecessary to use such a beam splitter that is constituted by glass members or the like stuck to each other. As a result, the apparatus of FIG. 2 can be reduced in cost and weight in comparison with that of FIG. 1.
The apparatus of FIG. 2 is, however, disadvantageous in that the shape of luminous flux imaged on the light-reception surface of an optical detector 6 is disturbed because not only astigmatism but also other aberrations are generated by the beam splitter 7 when the light is transmitted therethrough. Therefore these disturbances have a bad influence on the focusing error signal or the like. If the thickness of the beam splitter 7 is decreased in order to prevent this aberration from being generated, it is difficult to maintain good surface accuracy of the beam splitter 7 so that the shape of the focused beam on the optical disk 4 is disturbed and the disturbance has a bad influence on recorded information or information to be read. Further, the direction of the astigmatism generated by the beam splitter 7 is determined only by the inclination of the beam splitter 7, so that there are restrictions on the arrangement of the pickup and in the shape of the optical detector in providing separation of the direction of the astigmatism from the direction of detection of a tracking signal. These restrictions result in a reduction in the number of degrees of freedom in designing the apparatus.
Known is a further conventional pickup having such the structure shown in FIG. 3. In the drawing, light irradiated from a light source 1 is made to be substantially parallel light rays by a condenser lens 8. The light reflected by an optical disk 4 is transmitted through a parallel-plane beam splitter 7. Astigmatism and the like are hardly generated by the beam splitter 7 because the reflection light caused to be incident into the beam splitter 7 has been made to be parallel light rays. The light transmitted through the beam splitter 7 is converged by a condenser lens 9 and thereafter astigmatism is given to the converged light by a cylindrical lens 5.
In the thus arranged conventional apparatus, it is possible to eliminate the foregoing disadvantages in the conventional one of FIG. 2 because no aberration other than astigmatism is generated. However, the apparatus is disadvantageous in that the number of parts and portions to be adjusted are increased, resulting in increase in size, in cost, as well as in weight of the pickup.